This is directed generally to thermochromic coatings and inks and to documents, such as for example security documents, that incorporate such coatings and inks. The use of thermochromic inks for printing documents of various types is well known. Security documents, such as currency, checks and related negotiable instruments, as well as personnel identification devices, such as birth certificates, social security cards and driver's licenses, are all subject to being counterfeited. Various techniques have been used to make these sorts of documents difficult to replicate accurately. Some techniques are effective but require that the recipient of the document have special materials or equipment to authenticate documents. An example of such a technique is printing with special, chemically reactive inks which reversibly change color when subjected to solvent from a solvent pen, but which revert to their original color when dry. Other techniques, such as micro printed messages and ribbons of special material embedded in the document substrate, require special printing techniques and expensive paper or other document substrate material. Thermochromic inks printed on security documents have offered a method of verifying document authenticity which requires no special equipment or materials. When a security document printed with a thermochromic ink is warmed, such as by exposure to an individual's skin or breath, a color change will be observed and the document authenticated to some degree.
Other applications have been found for thermochromic inks and coatings. A thermochromic ink printed on the surface of a product, such as for example a drink container, may indicate to an observer whether the product is the proper temperature for use or consumption. Additionally, thermochromic inks have been used on novelty devices of various sorts.
The thermochromic inks previously in use have incorporated leuco dyes and have typically assumed one of two temperature dependent states—one in which the ink appears as a predetermined color and one in which the ink is substantially transparent or appears as a different color. This has been somewhat limiting. It might be desirable in some applications for a thermochromic ink to achieve more than two colors as the temperature of the ink is changed.